Peritoneal dialysis adequacy meaurements

ABSTRACT

The invention relates to devices, systems, and methods for measuring dialysis adequacy in patients undergoing peritoneal dialysis treatment. The devices, systems, and methods use one or more flow sensors and one or more solute sensors to measure the concentration of a uremic solute and volume of the peritoneal dialysate removed from the patient, and to calculate the dialysis adequacy based on the measured peritoneal dialysate concentration and volume.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/768,320 filed Nov. 16, 2018, the entire disclosure of which is incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to devices, systems, and methods for measuring dialysis adequacy in patients undergoing Peritoneal Dialysis (PD) treatment. The devices, systems, and methods use one or more flow sensors and one or more uremic solute sensors to measure the uremic solute concentration and volume of the peritoneal dialysate removed from the patient and to calculate the dialysis adequacy based on the measured peritoneal dialysate concentration and volume.

BACKGROUND

Peritoneal Dialysis (PD) is a dialysis treatment where a peritoneal dialysis fluid is cycled into and out of a peritoneal cavity to perform exchange across the peritoneum of the patient. Toxins and metabolic waste products are exchanged between the fluid injected into the peritoneum and the vascularized peritoneal membrane. To measure sufficiency of therapy, dialysis adequacy can be used as a performance metric. Dialysis adequacy is used to measure and to help ensure that patients are receiving a proper dose of dialysis therapy and to set a patient's peritoneal dialysis prescription. Known methods and systems commonly estimate dialysis adequacy by sending the removed fluid to a laboratory for analysis and either weighing an amount of the removed fluid or measuring the amount of fluid removed based on the time to drain the fluid and the flow rate. Alternatively, a blood urea concentration for the patient before and after treatment can be measured to determine the dialysis adequacy.

However, the known methods and systems fail to account for variation in determining dialysis adequacy. Due to the differences in performing PD and hemodialysis (HD), the corresponding clearance measurements can be different depending on the treatment modality. For example, in PD small solute clearance is usually measured by urea clearance normalized to total body water (Kt/V) or creatinine clearance normalized to body surface area, and includes a dialytic and a residual renal component. The residual renal component can be important in PD because the component can account for a significant proportion of the overall clearance achieved. In contrast, the residual renal component is oftentimes not considered in HD. Further, many PD patients have a more active and variable lifestyle, which may make standard assessments of urea clearance by blood draws and analysis more inconvenient. Known systems and methods do not account for these variations in clearances for a patient. Notably, the dialytic component is the only component that can be directly modified via a prescribing physician.

The known methods and systems also cannot provide a real-time or nearly-real time assessment of dialysis adequacy for peritoneal dialysis, nor can the known systems provide assessment of peritoneal membrane transport and/or ultrafiltration capability. If a patient undergoes HD, any residual kidney function quickly diminishes, but with PD residual kidney function can persist, making residual kidney function an important part of Kt/V measurements in PD. Moreover, the known systems and methods adjust patient prescription based on infrequently collected labs, typically once a month or less. As a result, changes in PD efficacy that occur between collections go undetected and untreated, leading to reduced overall effectiveness of dialysis. To measure Kt/V, known methods and systems require aliquots of spent peritoneal dialysate to be removed from a patient and then weighed to determine the total volume removed. For patients that have significant residual kidney function, such as patients producing >100 ml/day of urine, the urine must also be tested. The existing methods and systems require that the aliquot of the spent peritoneal dialysate be then physically transferred to a laboratory and then analyzed to determine the urea concentration. The process takes considerable time and is only performed periodically. The process is burdensome to the patient, because the patient needs to transport their waste PD fluid and urine to a clinic. The process is also costly to run. The known methods and systems also fail to take into account substantial natural intra-individual variation between clearance measurements, even if repeated clearance measurements are performed in the same patient for the same prescription. The variation is particularly marked for the residual renal component of clearance. Some of the variation may be accounted for by inaccuracies in collections of dialysate and urine, but other variations can result from day-to-day variation in urinary volume, peritoneal ultrafiltration, degree of equilibration, alterations in hydration, fluid intake, timing of exchanges, and tonicity of peritoneal fluids used. Therefore, periodic measurements of clearance as performed by the known systems and methods are not real-time, costly, and burdensome to the patient. Moreover, periodic measurements do not adequately characterize day to day variability for the patient. Because PD is generally performed at home instead of a dialysis clinic, there are fewer opportunities for the patient to interact with physicians or healthcare providers, resulting in fewer opportunities to test adequacy. Patient compliance to periodic testing may also be an issue with PD because patients do not come to a dialysis clinic as in hemodialysis (HD).

Hence, there is a need for systems and methods for measuring and calculating dialysis adequacy, and in particular Kt/V for peritoneal dialysis patients using sensors integrated into a PD system or method. The need includes calculating the dialysis adequacy in peritoneal dialysis in real-time or nearly real-time. The need further includes sensors positioned in the PD system to obtain measurements that can be used to determine dialysis adequacy. The need further includes sensors positioned in a catheter of a peritoneal dialysis system. The need includes a processor for determining dialysis adequacy based on sensor measurements. The need also includes avoiding human error in collecting samples, variations in day-to-day variation in urinary volume, peritoneal ultrafiltration, degree of equilibration, alterations in hydration, fluid intake, timing of exchanges, and tonicity of peritoneal fluids used. The need further includes automated systems and methods that do not require removing fluid samples, weighing the fluid samples, and physically sending the fluid samples for analysis to a laboratory. The need includes accounting for the residual renal component in determining dialysis adequacy. The need extends to systems and methods that can measure clearance and provide the dialysis adequacy of each peritoneal dialysis session or cycle. The need includes systems and methods for less expensive and burdensome clearance measurements to provide for more frequent measurements that better capture the patient's normal variability in clearance and to allow physicians to better manage the patient.

SUMMARY OF THE INVENTION

The first aspect of the invention relates to a system. In any embodiment, the system can comprise a catheter for removing peritoneal dialysate from a patient; a fluid line fluidly connected to the catheter, or a reservoir fluidly connected to the catheter; one or more flow sensors in any one or more of the catheter or the fluid line; one or more uremic solute sensors measuring a uremic solute concentration in a peritoneal dialysate removed from the patient; and a processor in communication with the one or more flow sensors and one or more uremic solute sensors, wherein the processor is programmed to calculate a clearance based on measurements obtained from the one or more flow sensors and one or more uremic solute sensors.

In any embodiment, the processor can be programmed to calculate a dialysis adequacy using an equation Kt/V; wherein K is equal to clearance, t is time, and V is a patient water volume.

In any embodiment, the system can comprise an osmotic agent sensor in the catheter, in the reservoir fluidly connected to the catheter, or in the fluid line fluidly connected to the catheter. In any embodiment, the osmotic agent sensor can be a glucose sensor.

In any embodiment, the processor can be programmed to estimate a peritoneal membrane transport capability for a patient based on an osmotic agent concentration in a peritoneal dialysate at three or more time points during a peritoneal dialysis cycle. In any embodiment, the osmotic agent concentration can be a glucose concentration.

In any embodiment, the processor can be programmed to calculate a total Kt/V for the patient using an equation: total Kt/V is equal to Kt/V from dialysis and Kt/V from residual kidney function.

In any embodiment, the processor can be programmed to calculate the Kt/V from residual kidney function using an equation: Kt/V from residual kidney function=U/P×L_(u)/V, wherein U is a uremic solute concentration in urine of the patient, P is a blood uremic solute concentration, and L_(u) is a volume of urine produced.

In any embodiment, the processor can be programmed to receive the uremic solute concentration in urine and volume of urine produced from the patient.

In any embodiment, the processor can be programmed to estimate the patient water volume based on a patient weight.

In any embodiment, the one or more uremic solute sensors can be positioned in the catheter.

In any embodiment, the one or more uremic solute sensors can be positioned in a fluid line or reservoir fluidly connected to the catheter.

In any embodiment, the processor can calculate a clearance based on measurements obtained from the one or more flow sensors and one or more uremic solute sensors in real-time or nearly real-time.

In any embodiment, the uremic solute can be selected from the group consisting of urea, creatinine, beta-2 microglobulin, uric acid, and combinations thereof.

In any embodiment, the uremic solute sensors can be selected from the group consisting of a urea sensor, a creatinine sensor, a beta-2 microglobulin sensor, a uric acid sensor, and combinations thereof.

The features disclosed as being part of the first aspect of the invention can be in the first aspect of the invention, either alone or in combination, or follow a preferred arrangement of one or more of the described elements.

The second aspect of the invention is drawn to a method. In any embodiment, the method can comprise the steps of: calculating a clearance based on a uremic solute concentration in a peritoneal dialysate removed from a patient measured by one or more uremic solute sensors and a volume of peritoneal dialysate removed from a patient measured by one or more flow sensors.

In any embodiment, the method can comprise the step of calculating a dialysis adequacy using an equation Kt/V; wherein K is equal to clearance, t is time, and V is a patient water volume.

In any embodiment, the urea clearance can be calculated by an equation clearance=F/P×L, wherein F is the uremic solute concentration in the peritoneal dialysate removed from a patient, P is a blood uremic solute concentration of the patient, and L is the volume of peritoneal dialysate removed from the patient.

In any embodiment, the uremic solute concentration in the peritoneal dialysate can be measured with a uremic solute sensor in a catheter, a fluid line fluidly connectable to the catheter, or a reservoir fluidly connectable to the catheter.

In any embodiment, the volume of peritoneal dialysate removed from the patient can be measured with a flow sensor in a catheter, a fluid line fluidly connectable to the catheter, or a reservoir fluidly connectable to the catheter.

In any embodiment, P can be calculated using an equation P=F/(T×D), wherein T is a time point during a peritoneal dialysis cycle and D is a diffusion coefficient for a patient.

In any embodiment, the uremic solute concentration of the peritoneal dialysate can be measured at an end of a peritoneal dialysis cycle, and P can be calculated using an equation P=total uremic solute/V; wherein the total uremic solute is calculated using an equation total uremic solute=F*L+F*(V−UF), wherein UF is an ultrafiltration volume for the peritoneal dialysis cycle.

In any embodiment, the method can comprise the step of calculating a Kt/V from residual kidney function.

In any embodiment, the Kt/V from residual kidney function can be calculated using an equation Kt/V from residual kidney function=U/P×L_(u)/V, wherein U is a uremic solute concentration in urine of the patient and L_(u) is a volume of urine produced.

In any embodiment, the method can comprise the step of calculating a total Kt/V for the patient using an equation total Kt/V is equal to Kt/V from dialysis and Kt/V from residual kidney function.

In any embodiment, the method can comprise the step of estimating a peritoneal membrane transport capability for the patient using an osmotic agent sensor in the catheter, a fluid line fluidly connectable to the catheter, or a reservoir fluidly connectable to the catheter. In any embodiment, the osmotic agent sensor can be a glucose sensor.

In any embodiment, the step of determining dialysis adequacy from dialysis can be performed by a processor of a peritoneal dialysis system.

In any embodiment, the uremic solute can be selected from the group consisting of urea, creatinine, beta-2 microglobulin, uric acid, and combinations thereof.

The features disclosed as being part of the second aspect of the invention can be in the second aspect of the invention, either alone or in combination, or follow a preferred arrangement of one or more of the described elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a peritoneal dialysis cycler for calculating the dialysis adequacy for a patient.

FIG. 2 is a flow chart illustrating a method of calculating a patient blood urea concentration.

FIG. 3 is a flow chart illustrating a method of calculating urea clearance for a patient.

FIG. 4 is a flow chart illustrating a method of calculating Kt/V for a patient.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used have the same meaning as commonly understood by one of ordinary skill in the art.

The articles “a” and “an” are used to refer to one or to over one (i.e., to at least one) of the grammatical object of the article. For example, “an element” means one element or over one element.

The phrase “based on” generally means using one or more inputs to add, delete, update, or change in any way another one or more, or same, variable or parameter due to, or because of, the one or more inputs.

“Beta-2 microglobulin” is a protein making up one chain of the major histocompatibility complex. As used herein, “beta-2 microglobulin” can refer to the protein in solution, or in any state of matter.

A “beta-2 microglobulin sensor” is a component capable of measuring a concentration of beta-2 microglobulin in a fluid, a gas, or combinations thereof. Such components of measuring beta-2 microglobulin concentration can be by direct methods quantifying the actual presence of beta-2 microglobulin, or indirectly by measuring the byproducts of beta-2 microglobulin, or by subsequent reaction with beta-2 microglobulin or beta-2 microglobulin's by products.

The term “blood uremic solute concentration” refers to the concentration of a uremic solute in the blood of a patient.

The terms “calculating” or to “calculate” refer to obtaining a value for a parameter using one or more mathematical equations.

A “catheter” is a single or plural lumen tube for flowing a fluid, gas, combinations of substances, solutes, and any combination thereof from a first location to another. For example, a catheter can introduce or remove fluid to a body cavity of a patient.

In general, the term “clearance” refers to an amount of a given substance removed from a patient. The fluid can be the blood of a patient during dialysis. In certain embodiments, clearance can be the amount of the substance removed from the patient as a fraction of the total amount of the substance in the patient.

The terms “communication” or “electronic communication” can refer to the ability to transmit electronic data, instructions, information wirelessly, via electrical connection, or any other electrical transmission between two components or systems.

The term “comprising” includes, but is not limited to, whatever follows the word “comprising.” Use of the term indicates the listed elements are required or mandatory but that other elements are optional and may be present.

The term “consisting of” includes and is limited to whatever follows the phrase “consisting of” The phrase indicates the limited elements are required or mandatory and that no other elements may be present.

The term “consisting essentially of” includes whatever follows the term “consisting essentially of” and additional elements, structures, acts or features that do not affect the basic operation of the apparatus, structure or method described.

“Creatinine” refers to C4H7N3O in solution, liquid, gaseous, or solid form.

A “creatinine sensor” is a component capable of measuring a concentration of creatinine in a fluid, a gas, or combinations thereof. Such components of measuring creatinine concentration can be by direct methods quantifying the actual presence of creatinine, or indirectly by measuring the byproducts of creatinine, or by subsequent reaction with creatinine or creatinine's by products.

The terms “determining,” “determines,” and the like, generally refer to, in the broadest reasonable interpretation, any process or method for obtaining or coming to a decision, value, number, or finding, for any one or more value, output, parameter, or variable, by any means applicable to the relevant parameter being determined.

The term “dialysis adequacy” refers to an amount, or dosage, of treatment by dialysis. Dialysis adequacy can refer to a measurement of the amount of solutes cleaned from the blood of a patient by dialysis therapy.

The term “diffusion coefficient” refers to the rate at which solutes pass through a membrane of a patient. The diffusion coefficient can be for a specific type of membrane, such as the peritoneum.

An “end of a peritoneal dialysis cycle” refers to a time point during peritoneal dialysis cycle during or just prior to draining the peritoneal dialysate from the patient.

“Estimated,” “estimating,” to “estimate,” or “estimation” can each refer to a determination of one or more parameters indirectly using one or more variables.

The term “flow sensor” refers to any component capable of measuring a volume or a rate of fluid moving through from a first point to a second point.

The term “fluidly connectable” refers to a capability for providing the passage of fluid, gas, or combination thereof, from one point to another point. The ability of providing such passage can be any connection, fastening, or forming between two points to permit the flow of fluid, gas, or combinations thereof. The two points can be within or between any one or more of compartments of any type, modules, systems, components, and rechargers.

The term “fluidly connected” refers to a particular state such that the passage of fluid, gas, or combination thereof, is provided from one point to another point. The connection state can also include an unconnected state, such that the two points are disconnected from each other to discontinue flow. It will be further understood that the two “fluidly connectable” points, as defined above, can from a “fluidly connected” state. The two points can be within or between any one or more of compartments, modules, systems, components, and rechargers, all of any type.

A “fluid line” can be any conduit or passageway that permits flow of a liquid, gas, or combination thereof from a first point to a second point.

The term “glucose concentration” refers to an amount of glucose dissolved in a given volume of solvent.

A “glucose sensor” is a component capable of measuring a concentration of glucose in a fluid.

“Kt/V from dialysis” is a measurement of dialysis adequacy based on dialysis treatment. In general, Kt/V is a ratio of the volume of fluid cleared of a solute divided by the distribution volume of the solute in a patient where the factor Kt represents the volume of fluid expected to be cleared of the solute during a specified period of time and V is a volume of distribution of the solute, approximately equal to patient's total body water, a volume of water in a patient prior to starting dialysis, for which Kt/V is measured. In particular, K can represent clearance of the solute or the volume of blood that is completely cleared of the solute as a function of time, and is oftentimes expressed in milliliters per minute (mL/min), and t represents time.

“Kt/V from residual kidney function” is a measurement of a solute removed from a patient by action of the patient's kidneys. The solutes removed by residual kidney function is generally removed in urine.

The term “measurement,” “measuring,” or to “measure” refers to determining a state or parameter of a system or substance. For example, a sensor can obtain measurements of a uremic solute.

An “osmotic agent” is a substance dissolved in water capable of driving a net movement of water by osmosis across a semi-permeable membrane due to concentration differences of the osmotic agent on each side of the semi-permeable membrane. Osmotic agents can include glucose, icodextrin, dextrose, and any other suitable substance or compound known to those of skill in the art for use as an osmotic agent in peritoneal dialysis.

The term “osmotic agent concentration” refers to the amount of one or more osmotic agents in a fluid per unit volume.

A “patient” or “subject” can be a member of any animal species, preferably a mammalian species, optionally a human. The subject can be an apparently healthy individual, an individual suffering from a disease, or an individual being treated for a disease. In certain embodiments, the patient can be a human, sheep, goat, dog, cat, mouse or any other animal.

The term “patient water volume” refers to the total amount of water within a body of a patient.

The term “patient weight” refers to the mass of a patient. The patient weight can either refer to an ideal mass of the patient, or the actual mass of the patient including any additional fluid in the body of the patient.

“Peritoneal dialysate” is a fluid solution to be used in peritoneal dialysis having specified parameters for purity and sterility and containing one or more solutes. Peritoneal dialysate is different than the dialysate used in hemodialysis.

“Peritoneal dialysis” is a therapy wherein a peritoneal dialysate fluid infused into the peritoneal cavity, which serves as a natural dialyzer. In general, waste components diffuse from a patient's bloodstream across a peritoneal membrane into the dialysis solution via a concentration gradient and through the fluid that is transferred to the peritoneum. In general, excess fluid in the form of plasma water flows from a patient's bloodstream across a peritoneal membrane into the dialysis solution via an osmotic gradient. Once the infused peritoneal dialysis solution has captured sufficient amounts of the waste components the fluid is removed. The cycle can be repeated for several cycles each day or as needed.

The term “peritoneal dialysis cycle” or “cycle” refers to the infusion of peritoneal dialysate into a patient, a dwell of the peritoneal dialysate within the peritoneal cavity of the patient, and the removal of the peritoneal dialysate from the peritoneal cavity of the patient. The process of filling and then draining your abdomen can also be seen as an “exchange” of used and clean fluids. However, the number, length, and timing of “cycles” or “exchanges” are non-limiting. For example, Continuous Ambulatory Peritoneal Dialysis (CAPD), Assisted Peritoneal Dialysis (APD) and Continuous Cycling Peritoneal Dialysis (CCPD) may occur on different schedules, but the process of filling and then draining the peritoneal cavity can be referred to as “cycles” for CAPD, APD, and CCPD. As such, the term is “cycle” or exchange refers to any particular dialysis schedule or type of dialysis.

A “peritoneal dialysis system” is a set of components for conducting peritoneal dialysis therapy. The peritoneal dialysis system can include components for introducing peritoneal dialysate into a patient, draining peritoneal dialysate from a patient, and optionally generating peritoneal dialysate.

The term “peritoneal membrane transport capability” refers to the ability of solutes to pass through the peritoneal membrane.

The term “positioned” refers to a component connected to or in contact with the feature being referred to. The contact can be physical, fluid, or electrical and is intended to be used in the broadest reasonable interpretation.

The term “processor” as used is a broad term and is to be given its ordinary and customary meaning to a person of ordinary skill in the art. The term refers without limitation to a computer system, state machine, processor, or the like designed to perform arithmetic or logic operations using logic circuitry that responds to and processes the basic instructions that drive a computer. In any embodiment of the first, second, third, and fourth invention, the terms can include ROM (“read-only memory”) and/or RAM (“random-access memory”) associated therewith.

The term “programmed,” when referring to a processor, can mean a series of instructions that cause a processor to perform certain steps.

The term “real-time” refers to decisions, determinations, or adjustments that are made concerning events that are ongoing as information is received or immediately after information is received. The terms “near real-time” or “nearly real-time” refers to decisions, determinations, or adjustments that can be made shortly after information is received.

The term “receiving” or to “receive” means to obtain information from any source.

The terms “removing” or to “remove” refer to withdrawing a substance from a container, conduit, or patient.

The term “residual kidney function” refers to the remaining ability of a patient's kidneys remove toxins and regulate solute and fluid levels in patients with kidney disease.

A “reservoir” can be a container or component that holds a liquid, fluid, gas, or combination thereof.

The term “sensor,” as used herein, can be a converter of any type that can measure a physical property or quantity of a matter in a solution, liquid or gas, and can convert the measurement into a signal which can be read by an electronic instrument.

A “time point during a peritoneal dialysis cycle” refers to a length of time from a beginning of a peritoneal dialysate cycle to a specific moment afterwards.

“Total Kt/V” refers to a sum of the Kt/V resulting from dialysis of any type and the Kt/V from residual kidney function for a patient, such as residual renal function from micturition.

The term “total uremic solute” refers to the sum of the amount of a uremic solute in the blood of a patient and the amount of the uremic solute in peritoneal dialysate inside or removed from the patient.

The term “ultrafiltration volume” refers to a volume of water removed from the blood of a patient during dialysis treatment.

The term “uremic solute clearance” refers to an amount of a uremic solute removed from the blood of a patient. For example, a peritoneal urea clearance can be calculated by determining a urea concentration in a removed peritoneal dialysate divided by a total blood urea concentration multiplied by the total peritoneal dialysate volume. However, the uremic solute clearance can be based on any uremic solute, and need not involve urea.

The term “uremic solute concentration” refers to an amount of a uremic solute dissolved in a given volume of solvent.

“Urea” refers to CO(NH2)2 in solution, liquid, gaseous, or solid form.

A “urea sensor” is a component capable of measuring a concentration of urea in a fluid, a gas, or combinations thereof. Such components of measuring urea concentration can be by direct methods quantifying the actual presence of urea, or indirectly by measuring the by products of urea, carbon dioxide and ammonia in any physical state, or by subsequent reaction with urea or urea's by products.

A “uremic solute” is a nitrogenous substance dissolved in a solvent. Non-limiting examples of uremic solutes can include urea, creatinine, beta-2 microglobulin, uric acid, and others.

A “uremic solute sensor” is a component capable of measuring a concentration of a specified uremic solute in a fluid, gas, or combination thereof. The component can measure the uremic solute concentration by direct methods quantifying the actual presence of the uremic solute, or indirectly by measuring the by-products of the uremic solute in any physical state, or by subsequent reaction with the uremic solute or the uremic solute's by-products. One non-limiting group of uremic solute sensors can be selected from the group of a urea sensor, a creatinine sensor, a beta-2 microglobulin sensor, and a uric acid sensor.

“Uric acid” refers to C5H4N403 in solution, liquid, gaseous, or solid form.

A “uric acid sensor” is a component capable of measuring a concentration of uric acid in a fluid, a gas, or combinations thereof. Such components of uric acid concentration can be by direct methods quantifying the actual presence of uric acid, or indirectly by measuring the byproducts of uric acid, or by subsequent reaction with uric acid or uric acid's by products.

The term “urine” refers to a fluid containing waste solutes removed from the body by the kidneys.

The term “urine produced” refers to a fluid generated by the kidneys containing solutes from the body.

The term “volume” refers to the three-dimensional space occupied by a substance or container.

Dialysis Adequacy Measurements

The invention is drawn to systems and methods for determining the adequacy or Kt/V of treatment for patients undergoing peritoneal dialysis. FIG. 1 illustrates a peritoneal dialysis system that can be used to determine the Kt/V from dialysis for peritoneal dialysis patients. Peritoneal dialysate from a peritoneal dialysate source (not shown) can be introduced into the peritoneal cavity of a patient 101 through fluid line 106. The peritoneal dialysate passes valve 105 and connector 104 into catheter 102 for infusion into the patient 101. Optionally, a filter 103 can be included to remove any particulate matter prior to infusion into the patient 101. After a dwell period, the fluid in the peritoneal cavity is removed from the patient 101 through catheter 102 into fluid line 107. Valve 105 controls the movement of fluid during fill and drain cycles. Pump 110 provides the driving force necessary for removing the peritoneal dialysate from the patient 101. In certain embodiments, the fluid line 107 can be fluidly connected to a reservoir 112 through connector 111. Alternatively, the fluid line 107 can be connected to a drain for disposal of the used peritoneal dialysate.

The peritoneal dialysis system can include one or more sensors for measuring parameters that can be used to determine Kt/V from dialysis. As illustrated in FIG. 1 , the sensors can include a flow sensor 108 and a uremic solute sensor 109. In certain embodiments, the uremic solute sensor 109 can be a urea sensor. Although urea is the most commonly used marker for measuring dialysis adequacy, any other uremic toxin such as creatinine, beta-2 microglobulin, or uric acid can be monitored including monitoring combinations of any one or more of the described markers. The uremic solute sensor can measure any known uremic solute concentration in the peritoneal dialysis fluid using any appropriate sensor known to those of ordinary skill in the art. For example, the uremic solute sensor 109 can be a creatinine sensor, a beta-2 microglobulin sensor, a uric acid sensor, or a sensor for any other known uremic toxin. In addition to being a urea sensor, the uremic solute sensor 109 can be any one of a creatinine sensor, a beta-2 microglobulin sensor, a uric acid sensor, or a sensor for any other known uremic toxin. The creatinine sensor can be accomplished by any number of electrochemical methods known to those of skill in the art. For example, a creatinine sensor can include a sensing element having a creatinine deiminase enzyme or a pH-indicating compound. The creatine sensor can have an electrode and/or an optical excitation assembly for illuminating the sensing element and/or an optical detection assembly to measure a parameter indicative of creatinine concentration. Measurement of beta-2-microglobulin can be accomplished with sensors having a sensing surface containing antibeta-2-microglobulin antibodies. Any other known beta-2-microglobulin sensors can be used. An example of a uric acid sensor can be a sensor having an enzyme uricase deposited on a substrate to measure uric acid. Other known uric acid sensors can be used. Optionally, additional sensors such as pressure sensors, glucose sensors, potassium sensors, calcium sensors, sodium sensors, magnesium sensors, conductivity sensors, or combinations thereof can be used to aid in the Kt/V calculations.

Although illustrated in FIG. 1 as positioned in fluid line 107, the uremic solute sensor 109 and flow sensor 108 can alternatively be positioned in the catheter 102, or in the reservoir 112 fluidly connectable to the catheter 102. The flow sensor 108 can be used to determine the volume of peritoneal dialysate removed from the patient 101 and can be avoided if the volume can be accurately measured inside reservoir 112. The uremic solute sensor 109 measures the dialysate uremic solute concentration for use in the Kt/V calculations. The sensors can be in communication with a processor (not shown) programmed to calculate the Kt/V from peritoneal dialysis. The processor can be part of the peritoneal dialysis system, or a separate component in communication with the sensors through wired or wireless communication.

Kt/V from peritoneal dialysis can comprise a peritoneal and a residual renal component. The residual renal component can be important in peritoneal dialysis because residual renal function can possibly account for a proportion of total clearance, depending on the patient and the duration of the first instance of treatment. As such, total Kt/V is equal to the sum of the peritoneal Kt/V and renal Kt/V. Typically, peritoneal Kt/V can be the 24-hour dialysate uremic solute nitrogen content/serum uremic solute nitrogen. Renal Kt/V can equal the 24-hour urine uremic solute nitrogen content/serum uremic solute nitrogen.

Kt/V is equal to the uremic solute clearance over the patient water volume. The patient water volume V can be estimated using the Watson formula, or 0.58×the ideal body weight of the patient, or alternatively estimated using any other formula, including patient height, patient weight, and/or gender. For example, the value V can be estimated using anthropometric formulas, including either Watson or Hume, based on age, sex, height, and weight. Estimates of V from the Watson formulas, when compared to a gold standard, such as deuterium oxide dilution, are, on average, slightly low but the discrepancy can vary substantially from patient to patient, especially in the obese. To determine the Kt/V from dialysis, the uremic solute clearance is required. EQ(1) provides a uremic solute clearance calculation using data from the flow sensor 108 and uremic solute sensor 109. Uremic solute clearance_(dialysis) =F/P×L  EQ(1)

Where F is the uremic solute concentration in the peritoneal dialysate removed from the patient, P is a blood uremic solute concentration of the patient, and L is the volume of peritoneal dialysate removed from the patient. The uremic solute concentration in the peritoneal dialysate removed from the patient can be measured by uremic solute sensor 109. The volume of peritoneal dialysate removed from the patient can be measured by flow sensor 108. The blood uremic solute concentration of the patient can be calculated or estimated. In certain embodiments, the blood uremic solute concentration can be measured by measuring the concentration of uremic solute in the dialysate as a function of time dϕ/dt and using Fick's Law, given in EQ(2).

$\begin{matrix} {\frac{\partial\varphi}{\partial t} = {D\;{\Delta\varphi}}} & {{EQ}(2)} \end{matrix}$

Using Fick's Law, the patient blood uremic solute concentration can be modeled and estimated. The diffusion coefficient (D) can be patient and time dependent. D can be measured for a given patient using Peritoneal Equilibration Test (PET). The PET test characterizes the transport properties of a patient's peritoneum, by measuring the clearance rate of urea, creatinine, glucose and protein. The peritoneal equilibration test (PET) is an assessment of peritoneal membrane transport function in patients wherein solute transport rates are assessed by the rates of the solute's equilibration between the peritoneal capillary blood and dialysate. The ratio of solute concentrations in dialysate and plasma (D/P ratio) at specific times during a dwell can signify the extent of solute equilibration wherein D represents the concentration of the solute in the dialysate and P represents the concentration of the solute in the plasma. The ratio D/P can be determined for any solute that is transported from the capillary blood to the dialysate and represents the fraction of the solute that is cleared across the peritoneal membrane. Creatinine, urea, electrolytes such as magnesium, potassium, and calcium, phosphate, and proteins are commonly tested solutes for clinical use.

The PET test is generally conducted at initiation of peritoneal dialysis to establish the type of membrane for a particular patient and to guide the physician in setting the patient's prescription based on the assessment. In certain embodiments, the PET test can be conducted regularly to take into account any changes in the diffusion coefficient for a patient over time. However, the PET test requires a blood test to determine the blood concentrations of each of the solutes. Alternatively, the changes in the diffusion coefficient can be monitored using a modified PET test, which measures glucose transport as a function of time. As described, the peritoneal dialysis system can optionally include an osmotic agent sensor such as a glucose sensor, allowing for use of the modified PET test and calculation of the diffusion coefficient D. Based on the diffusion coefficient for glucose transport, the system can estimate the peritoneal membrane transport capability. The estimated peritoneal membrane transport capability can be based on information of the relative changes in the ability of uremic solutes to cross the peritoneal membrane compared to past measurements as evidenced by the diffusion coefficient for glucose. Based on the peritoneal membrane transport capability obtained from the dialysate-to-plasma ratio of any uremic solute such as creatinine and the dialysate glucose concentration at the end of the test compared to the start, a real-time or nearly real-time assessment of a patient's peritoneal membrane transport and ultrafiltration capability can be provided that is more precise than the typical categories of low (L), low-average (L-A), high-average (H-A), and high (H) transporters.

In certain embodiments, the dialysate glucose concentration can be measured at preset intervals at specified time points during a peritoneal dialysis cycle. A small amount of peritoneal dialysate can be removed from the patient at each time point, and the concentration of an osmotic agent determined with an osmotic agent sensor such as a glucose sensor. The preset intervals can be every 30 minutes, every hour, every two hours, or any other interval of time. In certain embodiments, at least three time points can be used for the modified PET test. The ratio of the osmotic agent concentration such as the glucose concentration at time T to the glucose concentration initially in the peritoneal dialysate can be used to estimate the diffusion coefficient D, and thereby estimate the peritoneal membrane transport capability. In certain patients with blood glucose levels greater than 235 mg/dL transport can be slowed because the gradient between the peritoneal dialysate and the blood is lower. A correction factor can be developed for diabetic patients with poor glucose control to allow use of the modified PET test.

As described, an osmotic agent sensor such as a glucose sensor can be positioned in a catheter, in a reservoir fluidly connected to the catheter, or in a fluid line fluidly connected to the catheter. Any sensor capable of determining the osmotic agent concentration can be used. In certain embodiments, the osmotic agent sensor could be of the type used for diabetes control. At pre-set intervals, an aliquot of PD fluid can be withdrawn from the patient's peritoneum, and the osmotic agent concentration such as the glucose concentration measured to give a time-dependent concentration gradient for osmotic agent. The gradient can be a function of osmotic agent absorption across the peritoneal membrane and dilution from ultrafiltration. At the end of the dwell period, the total fluid volume can be measured to determine the ultrafiltration component, which could be mathematically subtracted to give a total glucose flux into the patient. Knowing the osmotic agent concentration gradient between blood and peritoneal fluid, the osmotic agent flux into the patient can be used to determine the diffusion coefficient D used in the Fick's diffusion equation.

With a known diffusion coefficient, the blood uremic solute concentration of a patient can be estimated. In certain embodiments, the uremic solute concentration of the peritoneal dialysate can be measured soon after the peritoneal dialysate has been introduced into the patient. In such cases, the patient blood uremic solute concentration can be given by EQ(3), where T is the time point during a peritoneal dialysis cycle. P=F/(T×D)  EQ(3)

Alternatively, the dialysate uremic solute concentration can be measured at the end of a peritoneal dialysis cycle, under assumption that F and P have equilibrated. In such cases, the uremic solute clearance can be calculated using EQ (4) Uremic solute clearance=F×dialysate volume (Infused+UF)  EQ(4) Where UF is an ultrafiltration volume for a peritoneal dialysis cycle. The patient blood uremic solute concentration is given by EQ(5) Uremic solute patient=F×(V−UF)  EQ(5) because the blood uremic solute concentration is assumed to be equilibrated with the dialysate uremic solute concentration. The total uremic solute is the sum of the patient blood uremic solute concentration and the uremic solute clearance. The patient pre-dialysis blood uremic solute concentration can be found with EQ(6) Total uremic solute/V=pre-dialysis blood level  EQ(6)

With a known patient pre-dialysis blood uremic solute level, the Kt/V from dialysis can be calculated as described. The processor of the invention can automatically calculate the Kt/V in real-time or nearly real-time. The systems and methods can calculate the Kt/V as information from the sensors as data is received. For example, as flow rate and uremic solute concentration values are received by the processor, the processor can immediately determine the Kt/V for that session.

An aliquot of peritoneal dialysate can be removed from the patient during a first portion of a cycle after the uremic solute in the blood has equilibrated with the dialysate. For example, an aliquot can be removed after a set time period, such as 10 minutes, after filling the peritoneal cavity with dialysate. The initial equilibrated uremic solute concentration can be used to determine the initial patient blood uremic solute concentration. As described, the patient blood concentration at the end of a session can be determined from the dialysate uremic solute concentration at the end of the session. Using the pre and post-session patient blood uremic solute levels, the uremic solute reduction ratio can be obtained. From the uremic solute reduction ratio, clearance can be determined using EQ(7). Kt/V from dialysis=−ln(1−URR)  EQ(7)

Certain peritoneal dialysis patients have some level of residual kidney function. In such patients, the total Kt/V is the sum of the Kt/V from dialysis and the Kt/V from urine production. In calculating the total Kt/V to determine dialysis adequacy, the contribution from urine production in patients with a higher degree of residual kidney function can be taken into account. In certain embodiments, the system can take into account the Kt/V contribution from urine production for patients with a predetermined level of urine production, such as 100 mL/day, while ignoring the Kt/V for patients with lower residual kidney function. The predetermined threshold can be set higher or lower than 100 mL/day of urine production, including between 10 mL/day and 200 mL/day, between 10 mL/day and 50 mL/day, between 25 mL/day and 75 mL/day, between 50 mL/day and 150 mL/day, between 75 mL/day and 125 mL/day, between 100 mL/day and 150 mL/day, or between 125 mL/day and 200 mL/day. In certain embodiments, the Kt/V from residual kidney function can be taken into account for patients with any volume of urine production greater than 0. The contribution from residual kidney function can be calculated using EQ(8). Kt/V from residual kidney function=U/P×L _(u) /V  EQ(8) where U is the concentration of uremic solute in urine of the patient and L_(u) is a total volume of urine produced. As described, the patient blood uremic solute concentration P and the patient water volume V can be estimated. As described herein, the processor of the invention can automatically calculate the Kt/V in real-time or nearly real-time. The systems and methods can calculate the Kt/V as information from the sensors as measurements are received.

The concentration of uremic solute in urine can be determined by any method known in the art. In certain embodiments, uremic solute sensors can be used to measure the urine concentration of uremic solute. Alternatively, test strips can be used. Test strips for measuring the uremic solute concentration in urine are commercially available. The patient can use a test strip to determine the concentration of uremic solute in the urine and the concentration can be received by the processor. The patient can manually input the concentration of uremic solute in urine to the processor using an interface. Alternatively, digital readings of the concentration of uremic solute in urine can be made and communicated to the processor through wired or wireless communication. In certain embodiments, the concentration of uremic solute in urine can be modeled or estimated based on historical measurements for the patient. The volume of urine produced L_(u) can be measured by the patient and received by the processor.

In certain embodiments, the uremic solute sensors of the dialysis system can be used to measure the uremic solute concentration in the urine. A sample of the patient's urine can be introduced to a flow path having a uremic solute sensor, such as the flow path illustrated in FIG. 1 . In certain embodiments, the uremic solute concentration in the urine can be measured once, or at infrequent intervals. The measured uremic solute concentration in the urine can then be used as an estimate of the uremic solute concentration in the urine at later points in time, avoiding the need to measure the uremic solute concentration in the urine each time while improving the measurements.

Alternatively, the Kt/V from residual kidney function can be provided as a quantitative index. For example, a large urine output can be given a value of 5, no input can be given a value of 0, and intermediate amounts of urine output can be given values of 1-4. One of skill in the art will understand that any values can be used in the quantitative index. Using a quantitative index simplifies ambulatory measurements so that the patient does not need to measure the amount of urine and the uremic solute concentration at home.

The total Kt/V for a patient can be determined by the processor using EQ(9). Total Kt/V=Kt/V for dialysis+Kt/V from urine  EQ(9)

Current guidelines suggest a weekly Kt/V of 1.7 or daily of 0.24 are recommended minimums. Based on the total Kt/V calculation, the patient or caregiver can make any necessary adjustments to the peritoneal dialysis prescription to ensure the total Kt/V meets the minimum recommendations.

FIG. 2 illustrates a flow chart for estimating a blood urea concentration of a patient. Although FIG. 2 illustrates the method for estimating a blood urea concentration, as described, the same method can be used for any uremic solute. In step 201, small amounts of peritoneal dialysate can be removed from the patient at preset time points during a peritoneal dialysis cycle. In step 202, the glucose concentration of the peritoneal dialysate removed from the patient at the preset time points can be received by the system using an osmotic agent sensor such as a glucose sensor in a catheter, a fluid line fluidly connected to the catheter, or reservoir fluidly connected to the catheter. Using the change in glucose concentration with respect to time, the diffusion coefficient for the patient can be calculated or estimated in step 203, and the peritoneal membrane transport capability estimated. The urea concentration in the peritoneal dialysate removed from the patient can be received in step 204 from a urea sensor in a catheter, a fluid line fluidly connected to the catheter, or a reservoir fluidly connected to the catheter. With a known or estimated diffusion coefficient and a known urea concentration in the peritoneal dialysate removed from the patient, the patient blood urea concentration can be determined in step 205 using EQ(3). One of skill in the art will understand that the order of steps 203 and 204 can be reversed, or that steps 203 and 204 can be performed simultaneously by a processor in the peritoneal dialysis system.

Alternatively, the urea concentration in the peritoneal dialysate removed from the patient at an end of a cycle can be received in step 206. An ultrafiltration volume for the cycle can be received by the system in step 207. The ultrafiltration volume for the cycle can be measured by subtracting a volume of peritoneal dialysate infused into the patient from the volume of peritoneal dialysate removed from the patient. The volumes infused into and removed from the patient can be measured with flow sensors in the catheter or fluid lines of the peritoneal dialysis system. In step 208, the patient water volume can be received. The patient water volume can be estimated using anthropometric formulas, including either Watson or Hume, based on age, sex, height, and weight. The total urea volume can be calculated in step 209 using EQ(6), if the blood and dialysate urea concentrations have equilibrated by an end of a cycle. Using the total urea concentration and the patient water volume, the initial patient blood urea concentration can be determined in step 205, as described. One of skill in the art will understand that steps 206-209 can be performed in any order or can be performed simultaneously by a processor in the peritoneal dialysis system.

The system can use either the method illustrated in steps 201-205 or the method illustrated in steps 206-210 to determine the patient blood urea concentration. In certain embodiments, the system can use both methods to determine the patient blood urea concentration with two independent methods. The system can use the values obtained from each method to check the accuracy or can average the values obtained from each method to obtain a more accurate blood urea concentration.

FIG. 3 illustrates a flow chart for calculating urea clearance using sensors in a peritoneal dialysis system. As described, any uremic solute can be used in calculating clearance using the same method. In step 301, peritoneal dialysate can be removed from a patient. In step 302, the volume of peritoneal dialysate removed from the patient can be received by a processor of the system. The volume of peritoneal dialysate removed from the patient can be measured by a flow sensor in a catheter or a fluid line fluidly connected to the catheter of a peritoneal dialysate system. In step 303, the urea concentration of the peritoneal dialysate removed from the patient can be received by the system. The urea concentration of the peritoneal dialysate removed from the patient can be measured by a urea sensor in a catheter, a fluid line fluidly connected to the catheter, or a reservoir fluidly connected to the catheter. In step 304, the blood urea concentration of the patient can be received by a processor of the system. As described, the blood urea concentration can be determined by analysis of the patient's blood, or by the method illustrated in FIG. 2 . Using the blood urea concentration of the patient, the volume of peritoneal dialysate removed from the patient, and the urea concentration in the peritoneal dialysate removed from the patient, the urea clearance for the patient can be calculated in step 305 using EQ(1).

One of skill in the art will understand that steps 302-304 can be performed in any order or can be performed simultaneously. For example, the blood urea concentration of the patient in step 304 can be received prior to or simultaneously to the urea concentration or volume of the peritoneal dialysate removed from the patient.

FIG. 4 illustrates a flow chart for calculating the total Kt/V for a patient. In step 401, the patient water volume can be received. In step 402, the urea clearance can be obtained. The urea clearance can be determined as illustrated in FIG. 3 . In step 403, the Kt/V from dialysis can be calculated using the urea clearance over the patient water volume. Optionally, for patients with significant residual kidney function, the system can receive the total volume of urine produced by the patient and the urea concentration of the urine produced by the patient in step 404. In step 405, the Kt/V from residual kidney function can be calculated using EQ(8). In step 406, the system can determine the total Kt/V for the patient by from both dialysis and residual kidney function using EQ(9). Although FIG. 4 illustrates calculating a total Kt/V using urea concentrations, any uremic solute can be used, including creatinine, beta-2 microglobulin, uric acid, or any other known uremic toxin.

One skilled in the art will understand that various combinations and/or modifications and variations can be made in the described systems and methods depending upon the specific needs for operation. Moreover, features illustrated or described as being part of an aspect of the invention may be used in the aspect of the invention, either alone or in combination, or follow a preferred arrangement of one or more of the described elements. 

We claim:
 1. A system, comprising: a catheter (102) for removing peritoneal dialysate from a patient; a fluid line (107) fluidly connected to the catheter, or a reservoir (112) fluidly connected to the catheter; one or more flow sensors (108) in the catheter; one or more uremic solute sensors (109) measuring a uremic solute concentration in a peritoneal dialysate removed from the patient; at least one glucose sensor measuring a glucose concentration in the peritoneal dialysate removed from the patient; and a processor in communication with the one or more flow sensors and the one or more uremic solute sensors, wherein the processor is programmed to calculate a clearance based on measurements obtained from the one or more flow sensors and the one or more uremic solute sensors; the processor is programmed to cause removal of a portion of dialysate to be removed from the patient during a peritoneal dialysis dwell period; and the processor further programmed to calculate a diffusion coefficient during the peritoneal dialysis dwell period based on data received from the at least one glucose sensor during the same peritoneal dialysis dwell period.
 2. The system of claim 1, wherein the processor is programmed to calculate a dialysis adequacy using an equation Kt/V; wherein K is equal to clearance, t is time, and V is a patient water volume.
 3. The system of claim 1, the processor programmed to estimate a peritoneal membrane transport capability for the patient based on the data received from the at least one glucose sensor at three or more time points during a peritoneal dialysis cycle.
 4. The system of claim 1, the processor programmed to calculate a total Kt/V for the patient using an equation: total Kt/V is equal to Kt/V from dialysis and Kt/V from residual kidney function.
 5. The system of claim 4, the processor programmed to calculate the Kt/V from residual kidney function using an equation: Kt/V from residual kidney function=U/P×L_(u)/V, wherein U is a uremic solute concentration in urine of the patient, P is a blood uremic solute concentration, and L_(u) is a volume of urine produced.
 6. The system of claim 5, the processor programmed to receive the uremic solute concentration in urine and volume of urine produced from the patient.
 7. The system of claim 1, the processor programmed to estimate the patient water volume based on a patient weight.
 8. The system of claim 1, wherein the one or more uremic solute sensors are positioned in the catheter.
 9. The system of claim 1, wherein the one or more uremic solute sensors are positioned in a fluid line or reservoir fluidly connected to the catheter.
 10. The system of claim 1, wherein the processor calculates a clearance based on measurements obtained from the one or more flow sensors and one or more uremic solute sensors in real-time or nearly real-time.
 11. The system of claim 1, wherein the uremic solute is selected from the group consisting of urea, creatinine, beta-2 microglobulin, uric acid, and combinations thereof.
 12. The system of claim 1, wherein the one or more uremic solute sensors is selected from the group consisting of a urea sensor, a creatinine sensor, a beta-2 microglobulin sensor, a uric acid sensor, and combinations thereof.
 13. A method, comprising the steps of: calculating a clearance based on a uremic solute concentration in a peritoneal dialysate removed from a patient measured by one or more uremic solute sensors (109) and a volume of peritoneal dialysate removed from the patient measured by one or more flow sensors (108) in a catheter; and calculating a diffusion coefficient during a peritoneal dialysis dwell period based on data received from at least one glucose sensor during the same peritoneal dialysis dwell period; wherein the data received from the at least one glucose sensor is a glucose concentration in a portion of peritoneal dialysate removed from the patient during the peritoneal dialysis dwell period.
 14. The method of claim 13, further comprising a step of calculating a dialysis adequacy using an equation Kt/V; wherein K is equal to clearance, t is time, and V is a patient water volume.
 15. The method of claim 13 or 14, wherein the clearance is calculated by an equation clearance=F/P×L, wherein F is the uremic solute concentration in the peritoneal dialysate removed from the patient, P is a blood uremic solute concentration of the patient, and L is the volume of peritoneal dialysate removed from the patient.
 16. The method of claim 15, wherein P is calculated using an equation P=F/(T×D), wherein T is a time point during a peritoneal dialysis cycle and D is the diffusion coefficient for the patient.
 17. The method of claim 16, further comprising a step of estimating a peritoneal membrane transport capability for the patient using an osmotic agent sensor in the catheter, a fluid line fluidly connectable to the catheter, or a reservoir fluidly connectable to the catheter.
 18. The method of claim 15, wherein the uremic solute concentration of the peritoneal dialysate is measured at an end of a peritoneal dialysis cycle, and wherein P is calculated using an equation P=total uremic solute/V; wherein the total uremic solute is calculated using an equation total uremic solute=F*L+F*(V−UF), wherein UF is an ultrafiltration volume for the peritoneal dialysis cycle.
 19. The method of claim 13, wherein the uremic solute concentration in the peritoneal dialysate is measured with the one or more uremic solute sensors (109) in the catheter, a fluid line (107) fluidly connectable to the catheter, or a reservoir (112) fluidly connectable to the catheter.
 20. The method of claim 13, further comprising a step of calculating a Kt/V from residual kidney function.
 21. The method of claim 20, wherein the Kt/V from residual kidney function is calculated using an equation Kt/V from residual kidney function=U/P×L_(u)/V, wherein U is a uremic solute concentration in urine of the patient and L_(u) is a volume of urine produced.
 22. The method of claim 20, further comprising a step of calculating a total Kt/V for the patient using an equation total Kt/V is equal to Kt/V from dialysis and Kt/V from residual kidney function.
 23. The method of claim 13, wherein the step of calculating the clearance is performed by a processor of a peritoneal dialysis system.
 24. The method of claim 13, wherein the uremic solute is selected from the group consisting of urea, creatinine, beta-2 microglobulin, uric acid, and combinations thereof. 